As recent mobile phones have continuously been reduced in size and thickness, it has become difficult to ensure enough space for disposing a speaker inside the mobile phone. On the other hand, the size of hearing aids needs to be small because the hearing aids need to be worn inside the external acoustic meatus and there is a high demand for visually unobtrusive hearing aids.
Therefore, it is desired to reduce the size of speakers for use in mobile electronic devices such as mobile phones and hearing aids.
Speakers of mobile phones include: receivers which reproduce received voices, external sounds, and the like; and micro speakers which reproduce ringtones, music signals, and the like. On the other hand, speakers included in hearing aids function simply as the receivers.
The receivers included in the mobile phones and the hearing aids need to reproduce the received voice, the external sound, and the like, and therefore it is necessary to reproduce voices so that vowels and consonants are distinctly identifiable. Thus, as compared to the micro speakers and the like, which are used to reproduce the ringtones, the music signals, and the like, the receivers are required to have excellent bass characteristic and be capable of wideband reproduction.
On the other hand, if a typical speaker, which includes an edge for supporting the outer circumferential part of a diaphragm, is reduced in size, the width of the edge is narrowed and the stiffness of the edge is increased, thereby degrading the bass characteristic.
As described above, the receivers included in the mobile phones and the hearing aids are required to satisfy the improvement in bass characteristic simultaneously with its size reduction, which are requirements conflicting with each other.
Patent Literature 1 discloses a conventional speaker that is different from the typical speaker described above and does not include the edge for supporting the outer circumferential part of the diaphragm. FIG. 23A is a front view of the conventional speaker disclosed in Patent Literature 1. FIG. 23B is a sectional view of the conventional speaker.
In FIG. 23A and FIG. 23B, reference numeral 1 designates a yoke, reference numeral 2 designates a center pole, reference numeral 3 designates a through-hole, reference numeral 4 designates a magnet, reference numeral 5 designates a yoke, reference numeral 6 designates a frame, reference numeral 9 designates a diaphragm, reference numeral 10 designates a coil bobbin, reference numeral 11 designates a voice coil, reference numeral 12 designates a coupling cone, reference numeral 13 designates a magnetic fluid, reference numeral 14 designates wires, reference numeral 15 designates an electric heating liquid, reference numeral 17 designates a closed container, reference numeral 19 designates a sound-absorbing material, and reference numeral 21 designates fixing bolts.
Patent Literature 1 discloses that “in an embodiment as shown, the coil bobbin 10 and each wire 14 are joined to each other in the middle between the sealing magnetic gap and a voice coil driving magnetic gap, and each wire 14 is wound one or more times around the coil bobbin 10. Additionally, each wire 14 is disposed in each of the four through-holes 20 provided in the yoke 5 in the radial direction. The tension of the wires 14 is adjusted by the fixing bolts 21. This fixing bolt 21 also prevents leakage of the electric heating liquid 15, and thus the electric heating liquid can be injected by loosening up the 21.”
As described above, the speaker disclosed in Patent Literature 1 uses the wires 14, instead of the annular-shaped edge, to support the diaphragm 9 so that the diaphragm 9 is vibratable. Patent Literature 1 also discloses that “the annular-shaped edge member is not required, and thus the bass distortion is reduced even with respect to large amplitude.”
Patent Literature 1 further discloses that the center of the coil bobbin 10 is held by the magnetic fluid 13, and the heat of the voice coil 11 is released through the magnetic fluid 13.